Jeepwalker
Senior Member
- Joined
- Sep 8, 2017
- Posts
- 3,231
- Reaction score
- 3,450
- Location
- WI
- Ram Year
- 2012 Reg Cab, 4x4
- Engine
- 5.7 Hemi
Hemi395, Thanks for the response. I'll see if I can get an image of LT Knock under some acceleration.
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I'd like to take credit for that very valuable post from @Hemi395 but it wasn't mine . I am learning this as he posts it as well.
Correct, in theory 87 is safe to run because the pcm will compensate by pulling timing. I just don't like how it has to see Knock first before it adjusts.I read through it all and pondered.
It seems Chrysler (it was Chrysler when Ram was designed) elected to have the PCM ignition timing settings ST & LT for a learning algorithm, much like transmission shift programs do.
The quick setting ST to react to preventing knock damage (how quickly I don't know, which is what I was asking for knock damage risk assessment), then the LT 'learns' to look for that conditional input the next time and holds the retar*ded timing for some pre-estimated longer time period, to prevent ST 'toggling' ret***** and advance so much.
This all seems like a normal adjustment algorithm design to me. So I'm back to the real question - 87 octane should do no harm to the engine - ASSUMING the PCM engineers worked with the mechanical engineers to make the timing ret*ard fast enough when knocking is detected.
You and other reports of being able to hear it does not give me great confidence they hit that parameter. Still too many design teams working in functional silos instead of cross-functional project teams. Ask me how I know.
Correct, in theory 87 is safe to run because the pcm will compensate by pulling timing. I just don't like how it has to see Knock first before it adjusts.
Correct, in theory 87 is safe to run because the pcm will compensate by pulling timing. I just don't like how it has to see Knock first before it adjusts.
Your ears are correct, the colder the motor is the less knock you'll get. In the winter I never see any timing pulled or hear any knock until its completely warmed upHave you ever hooked up your scan tool and then compared "ice cold" vs "heat soaked" or as close to those extremes as you can? My ears tell me that I never hear knock when it's cold, but when I do hear it it's when the engine is hot, the hotter it is the more knock I hear. I would love to see that backed up by a tool as "hearing things" is not always reliable in my case.
So I wonder then, how "long term" are the "lt" tables. For those who are getting knock, it might be better then to pick an octane and run with it permanently, instead of switching back and forth with towing etc. If it takes longer than a tank for the LT table to update then the silly thing is permanetly confused as we mix and match octanes.
Try 91 out , I am back to 91 full time after running 89 for a whole year like I have said in previous posts and my truck just feels strong, from shifting to acceleration, the difference is undeniable and very noticeable ...You guys are convincing me to go back to 89 full time.
Now tell me if 91 really benefits the Hemi compared to 89.
Did I mention how much I hate software? Gimmee back a screwdriver and actuator screw!
Try 91 out , I am back to 91 full time after running 89 for a whole year like I have said in previous posts and my truck just feels strong, from shifting to acceleration, the difference is undeniable and very noticeable ...
Also my MPGs went up almost 2 mpg which I don't mind at all lol .... it's going to benefit the "HEMI" no doubt ....
No ... 89 doesn't either ...Does your 91 have ethanol blend? If so, how much?
No ... 89 doesn't either ...
Amen, I miss the days of adjusting a screw on a carb and tweeking the distributor a bit to tune your motorYou guys are convincing me to go back to 89 full time.
Now tell me if 91 really benefits the Hemi compared to 89.
Did I mention how much I hate software? Gimmee back a screwdriver and actuator screw!
I agree, stick with one fuel. Unless they've updated the PCMs on newer trucks, the "learned" data from Knock is erased every time you the truck has a cold soak. When I ran 87 it would knock like crazy for a bit until LT started every time cold started. But mines a 2013 sp it's possible the newer trucks have a longer memorySo I wonder then, how "long term" are the "lt" tables. For those who are getting knock, it might be better then to pick an octane and run with it permanently, instead of switching back and forth with towing etc. If it takes longer than a tank for the LT table to update then the silly thing is permanetly confused as we mix and match octanes.
That is one of the reasons out here in California we get better mpg with 91 .... I do remember when working for Dish network and we went to the east coast and the 91 out there had ethanol and we were getting questioned about the amount of times we filled up the trucks and vans with gas lol all due to the ethanol blend ...Ethanol free gas is a whole different thread lol